1. Field of the Invention
The invention relates in general to a photolithography and etching process of a semiconductor fabrication process. More particularly, this invention relates to a method of forming an opening in a wafer layer.
2. Description of the Related Art
As the dimension of a semiconductor device becomes smaller and smaller, techniques such as phase shift mask (PSM) or optical proximity correction (OPC) mask has to be used to improve the transferred pattern from a photomask during the exposure process. However, when the pattern dimension is smaller than a half of the wavelength of the light source, the diffraction effect becomes significant. Especially for the photoresist layer patterns of contact hole/via hole, the profile cannot be effective improved using the phase shift mask.
On the other hand, as some fine correction of the photomask pattern is required for optical proximity effect, the fabrication of the photomask is tedious. When the pitch of the pattern is too small, there is no space for forming the assistant feature on the photomask. As a result, the photoresist layer pattern may be formed with a rounded profile. The resultant opening in the wafer layer is thus very likely to be rounded. The exact shape, cross sectional area and resistance of the contact hole/via hole are difficult to control.
A method of forming an opening in a wafer layer. At least two patterned photoresist layers are formed on a wafer layer. Using different photoresist layers, many openings are defined. The wafer layer is then etched to form the opening. Each photoresist layer has a parallel linear pattern such as parallel strips or an array of rectangular blocks. The photoresist layers are superposed in a way that spaces between the patterns for each photoresist layers overlapped with each other for form openings that expose the underlying wafer layers. The wafer layer exposed in the openings is then etched to form contact/via holes without rounded corners while the rounded profiles has been cancelled by the superposition of the photoresist layers.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.